Process for purification of acrylonitrile



United States Patent 1 3,117,988 PROCESS FQR PURIFECATION 0FACRYLGNITRELE William T. Bolieter, Texas City, Tex., assignor toMonsauto (Iheznical Company, St. Louis, Mo, a corporation of Delaware NoDrawing. Filed May 23, 1960, Ser. No. 31,238

4 Claims. (Cl. 260-465.?)

The present invention relates to the process for the purification ofacrylonitrile. More particularly, the present invention relates to aprocess for the removal of hydrogen cyanide from acrylonitrile.

In the process of preparing acrylonitrile by reacting acetylene andhydrogen cyanide such impurities as hydrogen cyanide, lactonitrile, andacetaldehyde are found in the product. The removal of any one or all ofthese substances is generally tedious and very ditficult. Such methodsas distillation, countercurrent water washing treatment with copperoxide, iron sulfates, etc., have been used to purify productacrylonitrile.

When crude acrylonitrile is purified by means of distillation, acomplete removal of the above-mentioned impurities is not accomplished.During distillation laconitrile easily undergoes decomposition as soonas the temperature is raised above 50 C. On decomposition HCN andacetaldehyde are formed. On the other hand, both of these componentsre-combine to form lactonitrile, and an equilibrium is reached asfollows:

CH CHOHCN:CH HCO+HCN The distilled product is once more contaminatedwith HCN, acetaldehyde, and lactonitrile.

It is an object of this invention to provide a process for thepurification of acrylonitrile. It is a more specific object of thisinvention to provide a method for the removal of hydrogen cyanide fromproduct acrylonitrile. Additional objects will become apparent from thedescription of the invention herein disclosed.

In fulfillment of the objects or" this invention it has been found thatwhen product acrylonitrile containing impurities comprising hydrogencyanide, acetaldehyde, and lactonitrile is subjected to treatment withan excess of a salt of a metal from the right hand column of group ll ofthe periodic table, e.g., zinc, cadmium, mercury, a stable group HEmetal cyanide complex is formed which can be separated with the removalof the free HCN and the HCN bound as lactonitrile. The acrylonitrile andacetaldehyde are readily removed and separated by distillation. The HCNmay then be recovered from the stable complex by addition of HCl to formthe chloride of the metal. The metal salts prefered in the practice ofthis invention are the salts of mercury.

To illustrate the effectiveness of this invention, the followingexamples are given. It is to be understood, of course, that theseexamples are not to be construed in any manner as limiting theapplications, conditions, or operation of the invention hereindisclosed.

Example I One hundred milliliters of acrylonitrile containing five partsper million of hydrogen cyanide were placed in a 250 milliliterdistillation flask. A small mount of sodium acetate was added to thesolution to promote dissociation of lactonitrile. To this was added 0.02gram of mercuric Patented Jan. 14, iii 84lacetate. The mixture was thenflash distilled. An analysis of the distillate showed the total cyanideconcentration (free hydrogen cyanide plus lactonitrile) to be less thanone part per million.

Example 11 One hundred milliliters of acrylonitrile containing approximately 3.5 weight percent of hydrogen cyanide is placed in a 250milliliter distillation flask. To this is added five milliliters of anaqueous solution that is 2 M in sodium acetate and 0.02 M in mercuricacetate. The mixture is then flash distilled to remove acrylonitrile andacetaldehyde. Analysis of the distillate indicated less than 500 partsper million hydrogen cyanide.

Example 111 One hundred milliliters of a crude acrylonitrile samplecontaining five parts per million of hydrogen cyanide and five parts permillion of acetaldehyde is placed in a distillation flask. To this isadded two milliliters of an aqueous solution containing 0.05 gram sodiumacetate and 0.02 gram of mercuric acetate. The mixture is distilled andthe acetaldehyde and acrylonitrile removed as distillate. An analysis ofthe distillate indicated less than one part per million hydrogencyanide.

Example IV On hundred milliliters of acrylonitrile containing five partsper million hydrogen cyanide is placed in a 250 milliliter distillationflask. A small amount of sodium carbonate is added to the solution topromote dissociation of lactonitrile. To this was added 0.02 gram ofzinc acetate. The mixture was then flash distilled. An analysis of thedistillate showed the total cyanide concentration (free hydrogen cyanideplus lactonitrile) to be less than 1.5 parts per million.

The compounds which may be used as complexing agents for the cyanide arethe salts of metals of the right hand column of group H of the periodictable. The metals included in this group are zinc, cadmium, and mercury.it is, of course, necessary that the compound be sufficiently soluble togive a metal ion concentration adequate to complex all of the cyanidepresent as hydrogen cyanide and lactonitrile. In the practice of thisinvention, the salts of mercury are the preferred reactants forcomplexing the cyanide. Non-limiting examples of group ll metal saltswhich may be used in the practice of this invention are acetates,nitrates, and sulfates. The preferred concentration of group 113 metalsalt present, assuming complete solubility is l to 20 parts by weight ofmetal salt pre part by weight of l-lCN.

in the practice of this invention it is beneficial to carry out thecomplexing reaction in a slightly alkaline solution, although notnecessary. This promotes the dissociation of lactonitrile so that all ofthe hydrogen cyanide may be removed from the acrylonitrile. Practicallyany compound may be used to bring about the desired alkalinity with thelimitation being that the compound be one which will not interfere inany way with the complexing reaction. Examples of such compounds aresodium carbonate, sodium acetate, sodium hydroxide, and sodium borate.The amount of the alkaline compound added must, of course, be sufiicientto catalyze the dissociation of lactonitrile and may range from 0.1 mg.to 10 mg. per gram of lactonitrile present.

What is claimed is: 2. The process of claim 1 wherein the metal ismercury.

1. Aprocess of purifying acrylonitrile containing hydro- 3. The processof claim 1 wherein the metal is zinc. gen cyanide and lactonitrile asimpurities which com- 4. The process of claim 1 wherein the metal saltis an prises removing said impurities by treating the impure acetate.

acrylenitrile in the presence of 0.1 to 10 mg. of an alkaline 5Referenees (Zited in the file of this patent compound per gram oflactorntrile present, with a salt of :1 metal selected from the groupconsisting of metal ace- UNITED STATES PATENTS tates, metal sulfates andmetal nitrates, the amount of 2,351,157 Semen June13, 1944 said saltused being in a Weight ratio to HCN of 1:1 2,656,376 Martino Oct. 20,1953 to 20:1, said metal being one selected from the group 10 2,784,219Couvillon Mar. 5, 1957 consisting of zinc, cadmium and mercury. 2,83,559 Mutter June 10, 1958

1. A PROCESS OF PURIFYING ACRYLONTRILE CONTAINING HYDROGEN CYANIDE ANDLACTONITRILE AS IMPURITIES WHICH COMPRISES REMOVING SAID IMPURITIES BYTREATING THE IMPURE ACRYLONTRILE IN THE PRESENCE OF 0.1 TO 10 MG. OF ANALKALINE COMPOUND PER GRAM OF LACTONITRILE PRESENT, WITH A SALT OF AMETAL SELECTED FROM THE GROUP CONSISTING OF METAL ACETATES, METALSULFATES AND METAL NITRATES, THE AMOUNT OF SAID SALT USED BEING IN AWEIGHT RATIO TO HCN OF 1:1 TO 20:1, SAID METAL BEING ONE SELECTED FROMTHE GROUP CONSISTING OF ZINC, CADMIUM AND MERCURY.